1. Field of the Invention
This invention generally relates to computing environments, and more specifically, the invention relates to data logout procedures in computing environments. Even more specifically, the preferred embodiment of the invention relates to a method and system well suited for generating a data logout command across a channel-to-channel connection in a computing environment.
2. Background Art
Channel-to-channel (CTC) adapters have been used for many years as a general-purpose communication mechanism between computer systems. For example, CTC adapters have been the principle mechanism for connecting multiple S/390 system and zSeries (offered by International Business Machines Corporation of Armonk, N.Y.) hosts together in homogeneous environments. S/390 and zSeries hosts can also be connected to other heterogeneous environments, such as IBM's RS/6000 and/or AS/400 systems. The CTC adapter is independent of the end users protocol, and has wide application in areas such as coupling of multiprocessor systems as well as in traditional communications protocol stacks (e.g., TCP/IP, SNA).
The CTC adapter provides a CTC control unit (CU) function within a channel residing in a channel subsystem. The CTC CU function provides control of CTC I/O devices. I/O device types include such components as peripheral cache memories, data storage units such as direct access storage devices (DASD), printers, tape recorders, and the like. From a programming viewpoint, control of a CTC I/O device is accomplished through a logical device (simply “device” hereinafter), each residing in a channel subsystem and associated with a CTC CU function.
When an I/O device is controlled through commands passing from a channel in one channel subsystem to a channel in another channel subsystem via a CTC CU function, the link between the channels is a CTC connection (or channel path) comprising one or more logical paths. When a system first becomes operational, typically at least one logical path is established and one or more CTC connections are initialized, thus allowing I/O commands to control devices.
One widely used channel-to-channel connection is the IBM Fiber connection, known more commonly as FICON, which provides high speed links used for attaching, for example, s/390 control units to s/390 systems, allowing interconnection of input/output (I/O) devices and host systems that are separated by tens of kilometers. For example, FICON may be used with the IBM Enterprise Storage Server. FICON is based on the industry-standard Fibre Channel architecture; consequently, the two have much in common. FICON is described in detail by C. M. DeCusatis, et al., in “Fiber optic interconnects for the IBM S/390 Parallel Enterprise Server GS, ” IBM Journal of Research and Development, Vol. 43, No. 5/6 (September/November 1999), pp. 807 828. IBM, FICON, IBM Enterprise Storage Server, and S/390 are trademarks of the International Business Machines Corporation, Armonk, N.Y.
FICON supports several topologies, including point-to-point and switched. In a switched topology, one link is connected from a FICON channel to a port on a switch, and another link is connected from a port on a FICON control unit to port on the same switch. Typically, switched topology is used to connect two or more hosts to two or more control units such as I/O devices.
FICON has proven to be a very successful and is widely used. It is nonetheless believed that FICON can be improved. For instance, it may be difficult to debug FICON channel-to-channel (FCTC) devices because two channels are involved in a single operation, and if is possible for the FCTC control unit functionality for the operation to be on either of the involved channels. The channel that possesses the control unit functionality is where decisions are made relative to what the active state is and how commands are answered. Logging the state of the channel with the CU function is advantageous to more expeditious debug of FCTC problems. While a mechanism exists (although not yet exploited by z/OS) to force a non-disruptive channel log on the local channel via a CHSC instruction, no such means exists to signal the attached channel to log out if the FCTC control unit function is across a physical link.